I got a bicycle basket (the kind that is supposed to be attached to the handlebars) and threw away the handlebar mounts and put it on the rack with zip ties and I tied it to the seat with zip ties as well... it handles my backpack with my change of clothes, wipes and asundries, and a thermos of coffee fine... haven't tried to max it out.
you can get one from a sporting goods store, or one of the xmart type places...